Salmon-derived nitrogen in terrestrial invertebrates from coniferous forests of the Pacific Northwest.

Hocking MD, Reimchen TE - BMC Ecol. (2002)

Bottom Line:
Invertebrates varied in delta13C but did not always vary predictably with trophic level or habitat.From 19.4 to 71.5% of invertebrate total nitrogen was originally derived from salmon depending on taxa, watershed, and degree of fractionation from the source.Enrichment of delta15N in the invertebrate community below the falls in conjunction with the absence of delta13C enrichment suggests that enrichment in delta15N occurs primarily through salmon-derived nitrogen subsidies to litter, soil and vegetation N pools rather than from direct consumption of salmon tissue or salmon tissue consumers.

Background: Bi-directional flow of nutrients between marine and terrestrial ecosystems can provide essential resources that structure communities in transitional habitats. On the Pacific coast of North America, anadromous salmon (Oncorhynchus spp.) constitute a dominant nutrient subsidy to aquatic habitats and riparian vegetation, although the contribution to terrestrial habitats is not well established. We use a dual isotope approach of delta15N and delta13C to test for the contribution of salmon nutrients to multiple trophic levels of litter-based terrestrial invertebrates below and above waterfalls that act as a barrier to salmon migration on two watersheds in coastal British Columbia.

Results: Invertebrates varied predictably in delta15N with enrichment of 3-8 per thousand below the falls compared with above the falls in all trophic groups on both watersheds. We observed increasing delta15N levels in our invertebrate groups with increasing consumption of dietary protein. Invertebrates varied in delta13C but did not always vary predictably with trophic level or habitat. From 19.4 to 71.5% of invertebrate total nitrogen was originally derived from salmon depending on taxa, watershed, and degree of fractionation from the source.

Conclusions: Enrichment of delta15N in the invertebrate community below the falls in conjunction with the absence of delta13C enrichment suggests that enrichment in delta15N occurs primarily through salmon-derived nitrogen subsidies to litter, soil and vegetation N pools rather than from direct consumption of salmon tissue or salmon tissue consumers. Salmon nutrient subsidies to terrestrial habitats may result in shifts in invertebrate community structure, with subsequent implications for higher vertebrate consumers, particularly the passerines.

Mentions:
Invertebrate groups varied in δ13C but did not always vary predictably with trophic level or habitat (Figure 2). Nested ANOVA analysis using δ13C indicated significant variability only in taxonomic groupings (F = 11.801; p < 0.001; R2 = 0.657), with all other levels insignificant. Relatively high δ13C values were observed in millipedes from both watersheds in salmon and non-salmon sites, most likely a reflection of inorganic carbon content. Multiple comparisons revealed trophic separation for spiders over carabid beetles in all sites (Table 2). Spiders were enriched over root feeders on the Clatse River above the falls and on the Neekas below the falls. Carabids and root feeders did not differ in their δ13C values. Carabid beetles collected on the Neekas River were the only group to demonstrate isotopic enrichment below the falls (p = 0.042). Spiders on the Clatse River were found to be higher in δ13C above the falls than below (p= 0.016).

Mentions:
Invertebrate groups varied in δ13C but did not always vary predictably with trophic level or habitat (Figure 2). Nested ANOVA analysis using δ13C indicated significant variability only in taxonomic groupings (F = 11.801; p < 0.001; R2 = 0.657), with all other levels insignificant. Relatively high δ13C values were observed in millipedes from both watersheds in salmon and non-salmon sites, most likely a reflection of inorganic carbon content. Multiple comparisons revealed trophic separation for spiders over carabid beetles in all sites (Table 2). Spiders were enriched over root feeders on the Clatse River above the falls and on the Neekas below the falls. Carabids and root feeders did not differ in their δ13C values. Carabid beetles collected on the Neekas River were the only group to demonstrate isotopic enrichment below the falls (p = 0.042). Spiders on the Clatse River were found to be higher in δ13C above the falls than below (p= 0.016).

Bottom Line:
Invertebrates varied in delta13C but did not always vary predictably with trophic level or habitat.From 19.4 to 71.5% of invertebrate total nitrogen was originally derived from salmon depending on taxa, watershed, and degree of fractionation from the source.Enrichment of delta15N in the invertebrate community below the falls in conjunction with the absence of delta13C enrichment suggests that enrichment in delta15N occurs primarily through salmon-derived nitrogen subsidies to litter, soil and vegetation N pools rather than from direct consumption of salmon tissue or salmon tissue consumers.

Background: Bi-directional flow of nutrients between marine and terrestrial ecosystems can provide essential resources that structure communities in transitional habitats. On the Pacific coast of North America, anadromous salmon (Oncorhynchus spp.) constitute a dominant nutrient subsidy to aquatic habitats and riparian vegetation, although the contribution to terrestrial habitats is not well established. We use a dual isotope approach of delta15N and delta13C to test for the contribution of salmon nutrients to multiple trophic levels of litter-based terrestrial invertebrates below and above waterfalls that act as a barrier to salmon migration on two watersheds in coastal British Columbia.

Results: Invertebrates varied predictably in delta15N with enrichment of 3-8 per thousand below the falls compared with above the falls in all trophic groups on both watersheds. We observed increasing delta15N levels in our invertebrate groups with increasing consumption of dietary protein. Invertebrates varied in delta13C but did not always vary predictably with trophic level or habitat. From 19.4 to 71.5% of invertebrate total nitrogen was originally derived from salmon depending on taxa, watershed, and degree of fractionation from the source.

Conclusions: Enrichment of delta15N in the invertebrate community below the falls in conjunction with the absence of delta13C enrichment suggests that enrichment in delta15N occurs primarily through salmon-derived nitrogen subsidies to litter, soil and vegetation N pools rather than from direct consumption of salmon tissue or salmon tissue consumers. Salmon nutrient subsidies to terrestrial habitats may result in shifts in invertebrate community structure, with subsequent implications for higher vertebrate consumers, particularly the passerines.